The present invention relates to methods for purification of aromatic polyethers, and more particularly to methods for purification of aromatic polyetherimides.
Various types of aromatic polyethers, particularly polyetherimides, polyethersulfones, polyetherketones, and polyetheretherketones have become important as engineering resins by reason of their excellent properties. These polymers are typically prepared by the reaction of salts of dihydroxyaromatic compounds, such as bisphenol A (BPA) disodium salt, with dinitroaromatic molecules or dihaloaromatic molecules. Examples of suitable dihaloaromatic molecules include bis(4-fluorophenyl)sulfone, bis(4-chlorophenyl)sulfone, and the analogous ketones and bisimides as illustrated by 1,3-bis[N-(4-chlorophthalimido)]benzene.
According to U.S. Pat. No. 5,229,482, the preparation of aromatic polyethers by displacement polymerization may be conducted in the presence of a relatively non-polar solvent, using a phase transfer catalyst which is substantially stable under the temperature conditions employed. Suitable catalysts include ionic species such as guanidinium salts. Suitable solvents disclosed therein include o-dichlorobenzene, dichlorotoluene, 1,2,4-trichlorobenzene and diphenyl sulfone. A particularly preferred solvent is a monoalkoxybenzene such as anisole as disclosed in U.S. Pat. No. 5,830,974.
It is desirable to isolate aromatic polyether from a reaction mixture free from contaminating species that may affect the polymer""s final properties in typical applications. In a typical halide displacement polymerization process contaminating species often include alkali metal halide and other alkali metal salts, residual monomer species, and residual catalyst species. For maximum efficiency of operation it is desirable to recover any solvent employed and other valuable compounds such as catalyst species, and to provide waste streams which do not contaminate the environment. In particular it is often desirable to recover alkali metal halide, especially sodium chloride, for recycle to a brine plant for production of sodium hydroxide and chlorine.
Many conventional techniques are used to purify polymer-containing organic solutions. For instance, extraction with water and settling by gravity in a mixer/settling tank have been used for removal of aqueous-soluble species. However, water extraction methods will not work when the water phase emulsifies with or does not phase separate efficiently from the organic phase. The particular case of polyethers in anisole solutions presents special difficulties when mixing with water and separating by settling. Pure water typically will not separate from anisole or polymer/anisole solutions after mixing near room temperature because the difference between water and anisole densities is very small. Under these conditions emulsions may form. Even if the first stage of extraction is performed under conditions where density differences drive the separation of water (for example, treatment with brine), the final stage of extraction of water-soluble species still requires use of relatively pure water, which is more prone to emulsification. The end result is that even though alkali metal halide and/or catalyst may be transferred to the aqueous phase, the entrained water in the emulsified organic phase prevents a high recovery of both the halide and the catalyst. High purity polymer solutions with minimal residual species may then be extremely difficult if not impossible to obtain. Variations in either temperature of operation in the range of between about 20xc2x0 C. and about 100xc2x0 C. or in polymer concentration may promote settling due to density differences, but the presence of surface-active functional groups on the polymer may still promote emulsification, particularly the presence of ionic end-groups such as phenoxide and/or carboxylate left uncapped from the polymerization process. Another constraint is that the time for separation of the aqueous and organic phases must be fast, preferably on the order of minutes, so that separation rates do not slow down production. A method is needed that minimizes emulsification and is relatively fast for phase separation of the water and organic phases.
Dry filtration via filters or membranes has also been employed for the removal of relatively large suspended solids from polymer-containing organic solutions. The advantage is that no process water is needed, but the disadvantage is that the filter type has to be chosen carefully to avoid a high pressure drop as the solids cake builds. Filtration is not feasible if the solid particles plug, blind, or go through the porous filter media. Easy back flushing of the filter is also required for fast turn-around and repeated use. Alkali metal halides, such as sodium chloride, are typically insoluble in organic solvents such as anisole, but such halides may be present as small suspended solid crystals that are difficult to remove by standard filtration methods. Furthermore, residual monomer species such as alkali metal salts of monomer or complexes of catalyst and monomer may also be present which often cannot be efficiently removed by filtration alone.
Because of the unique separation problems involved, new methods are needed for efficiently separating aromatic polyether products from contaminating species in relatively non-polar solvents which have density similar to that of water. Methods are also required for recycling the solvent and for recovering useful catalyst and alkali metal halide species from any final waste stream.
After careful study the present inventors have discovered methods for purifying aromatic polyethers prepared in water-immiscible solvents with densities close to that of water. These new methods also provide efficient recovery of solvent, alkali metal halide, and valuable catalyst species.
In one of its embodiments the present invention provides a method for purifying a mixture comprising (i) an aromatic polyether reaction product made by a halide displacement polymerization process, (ii) a catalyst, (iii) an alkali metal halide, and (iv) a substantially water-immiscible organic solvent with a density ratio to water in the range of between about 0.9:1 and about 1.1:1 measured at a temperature in the range of between about 20xc2x0 C. and about 25xc2x0 C., comprising the steps of:
(a) quenching the reaction mixture with acid; and
(b) extracting the organic solution at least once with water.
In another of its embodiments the present invention provides a method for purifying a mixture comprising (i) an aromatic polyether reaction product made by a halide displacement polymerization process, (ii) a catalyst, (iii) an alkali metal halide, and (iv) a substantially water-immiscible organic solvent with a density ratio to water in the range of between about 0.9:1 and about 1.1:1 measured at a temperature in the range of between about 20xc2x0 C. and about 25xc2x0 C., comprising the steps of:
(a) quenching the reaction mixture with acid; and
(b) adding water to the mixture to effect agglomeration of solid species comprising alkali metal halide; and
(c) separating the solid species by a solid separation method.
In another of its embodiments the present invention provides a method for purifying a mixture comprising (i) an aromatic polyether reaction product made by a halide displacement polymerization process, (ii) a catalyst, (iii) an alkali metal halide, and (iv) a substantially water-immiscible organic solvent with a density ratio to water in the range of between about 0.9:1 and about 1.1:1 measured at a temperature in the range of between about 20xc2x0 C. and about 25xc2x0 C., comprising the steps of:
(a) subjecting the mixture to at least one solid separation step;
(b) then quenching the mixture with acid; and
(c) extracting the organic solution at least once with water.
In still another of its aspects the present invention provides a method for purifying a mixture comprising (i) an aromatic polyether reaction product made by a halide displacement polymerization process, (ii) a catalyst, (iii) an alkali metal halide, and (iv) a substantially water-immiscible organic solvent with a density ratio to water in the range of between about 0.9:1 and about 1.1:1 measured at a temperature in the range of between about 20xc2x0 C. and about 25xc2x0 C., comprising:at least one filtration step, and at least one ion exchange step.